1. Field of the Invention
The present invention relates to an organic light emitting diode device and a method for fabricating the same, and more particularly, to an organic light emitting diode device which can obtain a sufficient margin and expand a design area of a driving transistor, by reducing the number of contact holes to minimize the area occupied by the contact holes, and a method for fabricating the same.
2. Description of the Background Art
A liquid crystal display with the features of light weight and low power consumption has been generally used as a flat panel display (FPD). However, the liquid crystal display is not a self light emitting device but requires backlight as an external light source. In addition, the liquid crystal display has technical limits in its brightness, contrast, viewing angle and size. Efforts have been actively made to develop a new FPD solving the aforementioned problems.
An organic light emitting diode device which is one of the new FPDs is a self light emitting device. Compared with the liquid crystal display, the organic light emitting diode device has an excellent viewing angle and high contrast, and the weight, thickness and power consumption of the organic light emitting diode device can be reduced by omitting a backlight.
The organic light emitting diode device attains low DC voltage driving and fast response speed. In addition, the organic light emitting diode device is made of a solid material to resist an external shock, has a wide temperature range, and cuts down the fabrication cost.
Especially, unlike a liquid crystal display or a plasma display panel (PDP), the organic electroluminescent device requires only deposition and encapsulation equipment in fabrication. Therefore, a fabrication process thereof is considerably simplified.
On the other hand, a passive matrix method without using a special thin film transistor (TFT) has been generally used as a driving method of the organic light emitting diode device.
In the passive matrix method, scan lines and signal lines cross each other to form a device in a matrix manner. The passive matrix method sequentially drives the scan lines by time to drive each pixel. Accordingly, instantaneous brightness equivalent to average brightness multiplied by a line number is necessary to acquire target average brightness.
In the passive matrix method, when the number of the lines increases, a higher voltage and current must be instantaneously applied, which accelerates deterioration of the device and increases the power consumption of the device. As a result, the passive matrix method is not suitable for a high definition large-size display.
In an active matrix method, a TFT for turning on and off a pixel is disposed at each pixel and operated as a switch. A first electrode is turned on/off in pixel units, and a second electrode facing the first electrode is used as a common electrode.
In addition, in the active matrix method, a voltage applied to the pixels is charged in a storage capacitor, for applying power until the application of a succeeding frame signal. Therefore, the driving state can be continuously maintained for one frame, regardless of a number of scan lines.
The active matrix method generates the same brightness even at a low current, and thus attains low power consumption and high precision and is suitable for a large-size display. The basic structure of the active matrix organic light emitting diode device will now be explained.
Although not illustrated, gate lines are formed on a substrate (not shown) in a first direction, and a data line and a power supply line are formed in a second direction crossing the first direction and isolated from each other at a predetermined interval, for defining one pixel area.
A switching TFT which is an addressing element is formed at the crossing point of the gate line and the data line. A storage capacitor is connected to the switching TFT and the power supply line. A driving TFT which is a current source element is connected to the storage capacitor and the power supply line. An organic electroluminescent diode is connected to the driving TFT.
In the organic light emitting diode device, when a current is supplied to an organic light emitting material in the forward direction, electrons and holes move to be recombined through a P-N junction between an anode electrode which is a hole supply layer and a cathode electrode which is an electron supply layer, and have smaller energy after the recombination than the separated electrons and holes. The organic light emitting diode device uses a principle of emitting light by the energy released during the recombination of the electrons and holes. The switching TFT serves to control the voltage and store the current source.
FIG. 1 is a layout view illustrating an interconnection structure between at least two transistors g in one pixel of the organic light emitting diode device described above. Referring to FIG. 1, a gate electrode 13 of a second TFT which is a driving transistor is isolated from a source electrode 17 of a first TFT which is a switching transistor at a predetermined interval.
A connection wiring 27 for connecting the gate electrode 13 of the second TFT to the source electrode 17 of the first TFT through first and second contact holes 23 and 25 is disposed on the gate electrode 13 of the second TFT and the source electrode 17 of the first TFT.
The method for interconnecting the two transistors will now be explained with reference to FIGS. 2A to 2F. FIGS. 2A to 2F are schematic cross-sectional views illustrating a contact process between the drain/source and the gate, taken along line II-II of FIG. 1.
Referring to FIG. 2A, the gate electrode 13 of the driving transistor is formed by depositing a conductive material layer on an insulation substrate 11, and selectively patterning the conductive material layer by a light exposure and development process using a light exposure mask. When the gate electrode 13 of the driving transistor is formed, a gate electrode (not shown) of the switching transistor is also formed.
As illustrated in FIG. 2B, a gate insulation film 15 is deposited on the whole substrate 11 including the gate electrode 13. An active layer (not shown) and an ohmic contact layer (not shown) are sequentially laminated on the gate insulation film 15. A conductive material such as Mo is deposited on the whole substrate 11. The source/drain electrodes 17 of the first TFT which is the switching transistor are formed by selectively patterning the conductive material layer by a light exposure and development process using a light exposure mask. When the source/drain electrodes 17 of the first TFT are formed, the source/drain electrodes (not shown) of the driving TFT are also formed.
As shown in FIG. 2C, a protection film 19 is formed on the gate insulation film 15 including the source electrode 17 of the first TFT. As depicted in FIG. 2D, a photoresist film pattern 21 is formed to expose the protection film 19 on the source electrode 17 and the gate electrode 13 of the second TFT, by coating a photoresist film (not shown) on the protection film 19, and selectively removing the photoresist film by a light exposure and development process using a light exposure mask.
As illustrated in FIG. 2E, the first contact hole 23 and the second contact hole 25 are formed by selectively removing the protection film 19, the gate insulation film 15 and the source electrode 17 using the photoresist film pattern 21 as a mask. When the first contact hole 23 is formed, the source electrode 17 is etched to expose the gate insulation film 15 disposed below. When the second contact hole 25 is formed, the etching process is carried out to expose the top surface of the gate electrode 13.
Referring to FIG. 2F, the connection wiring 27 for electrically connecting the source electrode 17 of the first TFT to the gate electrode 13 of the second TFT is formed by removing the photoresist film pattern 21, depositing a conductive material layer on the whole substrate 11 including the first and second contact holes 23 and 25, and selectively patterning the conductive material layer.
The conventional organic light emitting diode device has the following disadvantages. In the related art active matrix organic light emitting diode device, especially, based on amorphous silicon, two or more transistors exist in one pixel, and the contact holes are formed to interconnect the transistors or the layers. In the case of a small-sized device, the pixel size is relatively small, and thus the area occupied by the transistors is limited in the pixel. Therefore, a current stress applied to the driving transistor increases. Accordingly, the lifespan of the transistor is shortened. That is, when the design area of the driving transistor is reduced, the lifespan of the transistor is relatively shortened. As a result, the lifespan of the organic electroluminescent device is shortened.